Various methods and apparatus for forming a polymer into particles are known. For example, polymers have been formed into solidified strands, ribbons, or sheets which have then been broken into particles. Fracturing or granulation of a sheet, for example, into particles may be accomplished by various methods including ball milling. Such methods of particle formation, however, may result in particles which are not uniform in size and shape. Furthermore, such methods may generate an undesirable amount of fines, which make particle handling and processing difficult.
It is also known to form polymer particles by first forming polymer drops, from a "melt" of the polymer, and subsequently solidifying the drops into particles or pellets. Pastillation is an example of one such method. For example, Chang et al., U.S. Pat. No. 5,340,509, disclose a process for pelletizing ultra high melt flow crystalline polymers which are polyolefin homopolymers, copolymers, or blends thereof. The process of Chang et al. uses, as a droplet-forming means, a pastillator which comprises an outer container with orifices. The outer container rotates around an inner container to allow a uniform amount of the polymer melt to emerge as droplets. The droplets are collected on a conveyor, which cools the droplets for a time sufficient to solidify the droplets.
In addition to pastillation methods, the formation of polymers into particles via polymer droplets has been accomplished in a variety of other ways. For example, U.S. Pat. No. 4,340,550 to Ho, discloses the preparation of free-flowing pellets of poly(ethylene terephthalate) oligomer by quenching droplets of molten oligomer in water. The molten oligomer is fed to a droplet-forming means having an orifice plate with multiple orifices. Under pressure, molten oligomer flows through the orifices and out into an inert gas. The molten oligomer dissociates into droplets at a distance from the plate under the force of surface tension. The molten droplets are then quenched in a tank of water. The oligomer pellets are slightly flattened, about 0.3 to 2.0 mm thick and about 0.8 to 4.0 mm in circular diameter.
Rinehart, U.S. Pat. No. 4,165,420, discloses forming particles by employing a spray congealer which forms particles from low-viscosity molten polymer. Molten polymer is conveyed to the rotating bowl of a centrifugal atomizing device. This device produces small spherical droplets which congeal, in an inert gas, in the form of spherical beads having an average particle size of 100-250 microns, depending on the speed of rotation of the bowl.
Uniform, crystalline, low molecular weight polyester particles, in size ranges suitable for mass handling (e.g., about 2 mm to 6 mm) are difficult to produce using traditional methods or apparatus for various reasons. A low molecular weight polyester, also referred to as an oligomer or prepolymer, when in a molten state, may have relatively low viscosity. Such low viscosity may cause difficulties in the formation of droplets of uniform shape and size, especially by conventional means because of the resulting low pressures.
Low molecular weight polyester particles, as produced by conventional methods, have the disadvantage that they may not be in a form most conducive to solid-state polymerization (SSP), especially in the absence of a time-consuming annealing step. Solid-state polymerization is used in industry to obtain quality-grade polycondensation polymers of very high molecular weight. Such solid-state polymerization typically involves heating a "prepolymer" which is a medium molecular weight polymer, in the form of chips. This polymer is heated to a temperature above its glass transition temperature (T.sub.g) but below its melting point (T.sub.m). In comparison, relatively low molecular weight particles, as feedstocks, may be disfavored because of the difficulty in forming such particles and because of the brittle nature of the formed particles.
Since polymerization reaction rates increase with temperature, the optimum temperature for solid-state polymerization is usually as close to the melting point as possible. In order to reduce sticking together at such high temperarture, polyester particles produced by conventional particle formation methods and apparatus typically need conditioning prior to solid state polymerization. Such conditioning may involve annealing at fairly high temperatures (e.g., 150.degree. C. to 210.degree. C. for polyethylene terephthalate) and for long amounts of time (e.g., about 0.5 to 8 hours). Such conditioning increases the crystallinity level of the particles. Typically, the pellets are initially subjected to a certain amount of annealing under high turbulence and agitation in order to achieve uniform annealing without sticking together. If such particles are not properly conditioned prior to solid-state polymerization, processing problems may result. For example, they may tend to stick together during SSP, resulting in an inability to discharge the particles from the SSP reactor, which may even result in a reactor shut-down.
As mentioned above, polyester particles or pellets formed by conventional methods may be unduly non-uniform, malformed and/or characterized by high levels of fines. Such malformed and non-uniform pellets may be undesirable because they may bridge in pellet feed hoppers. Additionally, significant amounts of malformed pellets may alter the bulk density of the pellet feedstock to a polymerization processes, which may result in feeding problems in extrusion lines. It may also result in voids in the final product. Since reaction rate is to some degree dependent on particle size, non-uniform pellets may result in non-uniform molecular weight in the product of polymerization.
In view of the above, there is a need for an improved process of forming polyester particles. In order to be useful as feedstocks for polymerization processes, such particles should preferably have sufficient structural integrity to make them suitable for transport to such processes. The particles should preferably have relatively uniform size and shape in order to facilitate handling and to ensure uniform polymerization within each particle. For improved solid-state polymerization, the particles should preferably be robust enough to withstand high temperatures during solid-state polymerization without agglomerating. It would be even more desirably if the particles could withstand higher temperatures than most typical of solid-state polymerization. It would be desirable if robust or crystalline particles could be obtained more efficiently and readily than presently the norm. Accordingly, it would be advantageous if costly and expensive steps for conditioning polyester particles prior to solid-state or other polymerizations could be reduced or eliminated. It would be desirable if such particles could have a diversity of uses, including, not only serving as a prepolymer or feedstock for solid-state polymerization, but optionally or additionally as feed material for, among examples, injection molding, bottle manufacture, and extrusion processes.